This section provides background information related to the present disclosure which is not necessarily prior art.
In recent years, transmissions have been developed featuring a large number of forward gears to allow for engine downsizing due to an increasing desire for improved fuel economy in automobiles and other vehicles. Smaller engines coupled to seven, eight, and nine speed transmissions are now common place. While a decrease in combustion volume leads to a proportional decrease in the size and mass of an engine, the volume of oil required does not decrease as much. Shorter residence times for the oil and higher engine speeds associated with smaller engines increase the probability of oil aeration, which can have a negative effect on the durability of the engine. Friction losses related to viscous drag also become more significant in smaller engines because engine downsizing typically leads to an increase in the thermal efficiency of the engine.
To compensate for the increase in oil aeration probability and frictional losses related to viscous drag, oil volume cannot be decreased proportionally with engine size. That is, smaller engines require proportionally more oil volume compared to larger engines. Smaller engines also deliver less torque to the drivetrain and generate less heat. Because smaller engines apply less thermal energy to proportionally more oil, it takes considerably longer for the engine to warm up the oil circulating through the engine and other drivetrain components such as the transmission.
Newer engines are also being equipped with engine start/stop features to improve fuel economy and emissions during city driving. Engine start/stop features automatically turn the engine off when the vehicle is brought to a rest and then automatically start the engine again when the accelerator is pressed and travel is resumed. With the engine turned off when the vehicle is at rest, oil is not circulating in the engine or the transmission leading to a longer period of time before the oil is warmed up to ideal operating temperatures. In other words, when the engine duty cycle includes multiple relatively short running cycles, the temperature of the oil in the engine and the transmission rises more slowly and a greater part of the engine's run time is spent with oil temperatures being below ideal operating temperatures. This is problematic because the viscosity of the oil that circulates through the engine and drivetrain varies with temperature. Specifically, the viscosity of such oils generally decreases (become less resistant to flow or is “thinner”) as temperature increases. Low viscosity is generally favored provided that sufficient lubricity is maintained because high viscosity (where the oil is more resistant to flow or is “thicker”) leads to an increase in viscous drag related losses and an attendant decrease in efficiency. These losses off-set much of the efficiency gains that can be realized through engine downsizing and engine start/stop features. Short engine running cycles also may not allow for the appropriate evaporation of water and fuel out of the oil contained in the oil sump of the engine, potentially leading to problems with oil overflow, excessive aeration, additional churning losses, and restrictions in the oil pickup due to ice crystals forming on the pickup inlet screen during cold weather operation.
Recently, new oil sumps for engines have been designed to address these problems. One such oil sump design, also developed by the inventor to the present disclosure, is disclosed in U.S. Patent Application Publication 2013/0312696 entitled “Temperature-Controlled Segregation of Hot and Cold Oil in a Sump of an Internal Combustion Engine.” The oil sump disclosed in this reference includes a porous separator disposed in the oil sump for separating the oil sump into hot and cold oil volumes during an engine cold start. The porous separator is arranged to create a trough-like volume around an oil pickup. The engine oil in this trough-like volume is isolated from the cold engine oil disposed in the rest of the oil sump until the temperature of the cold engine oil is raised to a temperature where its viscosity permits passage through orifices in the porous separator. Accordingly, the temperature of the circulating engine oil can be raised in a quicker manner after engine cold starts. However, the porous separator of this design relies on the temperature dependence of the viscosity of the engine oil to effectively function. The diameter of the orifices in the porous separator is selected to work with a particular engine oil based on its viscosity at a particular temperature threshold. This limits the flexibility of the design since only certain engine oils can be used and the temperature threshold of any given oil/orifice size combination cannot be changed. Other drawbacks arise from the structure of the porous separator itself, which could become clogged with large particulates suspended in the engine oil or by ice crystals forming in the orifices of the porous separator.
An alternative engine oil sump design is disclosed in U.S. Patent Application Publication 2011/0011367 entitled “Apparatus and Method for Rapid Warming of the Oil in an Oil Pan of an Internal Combustion Engine.” The engine oil sump disclosed in this reference includes a fixed gap disposed between a baffle and the wall of the oil sump that controls the flow of the engine oil depending on its viscosity. This reference suggests finding a gap size that effectively provides separation of cold and hot engine oil and that does not restrict maximum flow of the engine oil at elevated temperatures. However, for many engine oils, finding such a gap size may not be feasible. Either separation of the hot and cold engine oil will not be effective or the gap size will be too small and will restrict the maximum flow of the engine oil at elevated temperatures. Restricting the maximum flow of engine oil at elevated temperatures may lead to pickup starvation, aeration, and, thus, possible damage to the engine. If, on the other hand, the gap size is too large, the temperature of the engine oil will not rise fast enough to have an effect on the efficiency of the engine.
What is needed is a baffle design that can universally control the separation of hot oil and cold oil in an oil sump regardless of the particular oil that is used and that does not restrict oil flow within the oil sump at elevated temperatures.